Conductor crossover for a semiconductor detector

ABSTRACT

The invention relates to a conductor crossover for a semiconductor detector, particularly for a drift detector for conducting X-ray spectroscopy. The conductor crossover comprises at least two doped semiconductor electrodes ( 2 ), which are placed inside a semiconductor substrate ( 1 ), at least one connecting conductor (M), which is guided over the semiconductor electrodes ( 2 ), and a first insulating layer (Ox). An intermediate electrode (L) is situated between the connecting conductor (M) and the first insulation layer (Ox). Said intermediate electrode overlaps the area of the semiconductor substrate ( 1 ) between the semiconductor electrodes ( 2 ) and is electrically insulated from the connecting conductor (M) by at least one additional insulation layer (I). The invention also relates to a drift detector equipped with a conductor crossover of this type and to a detector arrangement for conducting X-ray spectroscopy.

The invention relates to a conductor crossover for a semiconductordetector, in particular for a drift detector for X-ray spectroscopy,according to the preamble of claim 1.

Semiconductor detectors for detecting electromagnetic radiation arebased on the measurement of free charge carriers which are generated bythe radiation in a semiconductor component. The charge carriers producean electrical signal, which is conducted over at least one connectingconductor to a measuring device. The design of the connecting conductorrepresents a general problem in the production of semiconductors. Thedetector together with the connecting conductor must be practicable tomanufacture and easy to handle, but must also not impair the detectorfunction via the connecting conductor.

There are, for example, semiconductor detectors for locally resolvedradiation detection which have multiple semiconductor electrodessituated in a semiconductor substrate (known as strip or trackdetectors). In these detectors, insulated connecting conductors areguided directly over the semiconductor electrodes to a substrate edge.However, such a conductor crossover is possible only when there is nointerfering difference of potential between the connecting conductor andthe semiconductor electrodes.

Drift detectors are known for high-resolution radiation detection in thefield of X-ray spectroscopy (see L. Strüder in “Nuclear Instruments andMethods in Physics Research A,” Vol. 454, 2000, pp. 73-113, and DE 34 27476 A1, for example). A drift detector comprises, for example, asemiconductor substrate made of weakly n-doped silicon which has astrongly n-doped, centrally located anode on one surface (top side), andhas a reverse contact made of a strongly p-doped semiconductor materialon the opposite surface (bottom side). Annular semiconductor electrodesmade of a strongly p-doped semiconductor material, concentricallyarranged about the anode are also provided on the top side. Thesemiconductor electrodes are each kept at a fixed electrical potentialwhich becomes increasingly negative as the distance from the anodeincreases. In the effective region of the electrodes this results intotal depletion of the semiconductor substrate, and also produces anelectrical drift field in the semiconductor substrate. Radiationinteractions cause free electrons to be generated in the semiconductorsubstrate which are driven through the drift field to the anode, so thatthe electrical signal at the anode is a measure of the energy and/orintensity of the radiation.

The design of the connecting conductor to the anode has thus farrepresented a significant problem in drift detectors. On account of thelarge differences in potential between the connecting conductor and thesemiconductor substrate together with the semiconductor electrodes, theabove-referenced guiding of the connecting conductor over thesemiconductor electrodes would lead to erroneous signals caused byundesired charge carrier amplification, and/or to electricalbreakthroughs. The known silicon drift detectors are thus characterizedby a freely guided contacting of the anode from the top side of thesemiconductor substrate to the outside. Electrical connecting conductorsare guided from the anode to a spatially separate connector unit ormeasuring device through the half-space adjoining the top side.

The free guiding of connecting conductors has several disadvantages. Theelectrical contacting is difficult since the connecting conductors aretypically attached by ultrasound bonding, whereby the semiconductorsubstrate tends to undergo undesired mechanical vibrations. As a result,faulty contacting may occur. Damage in the sensitive region of thesemiconductor substrate upon bonding may cause the detector to fail.Furthermore, the free guiding of connecting conductors has disadvantageswith regard to the stability of the contacting and the combination ofdrift detectors in groups.

The object of the invention is to provide an improved conductorcrossover for a semiconductor detector which eliminates thedisadvantages of conventional conductor crossovers and which may inparticular be used when there are large differences in potential betweena connecting conductor and semiconductor electrodes, which occur forexample in the drift detector described above.

This object is achieved based on a conductor crossover according to thepreamble of claim 1, and by the characterizing features of claim 1.

A basic concept of the invention is to provide a conductor crossover fora semiconductor detector having a semiconductor substrate andsemiconductor electrodes, whereby the conductor crossover is used toguide at least one connecting conductor away from, for example, asignal-emitting readout electrode to a measuring device via thesemiconductor electrodes and the semiconductor substrate, or via atleast one adjacent detector, and at least one intermediate electrode issituated between the connecting conductor and the semiconductorsubstrate. The intermediate electrode is electrically insulated relativeto the connecting conductor and the adjoining semiconductor substrate,and electrically shields at least one partial region of thesemiconductor substrate between the semiconductor electrodes from theconnecting conductor.

The provision of the intermediate electrode results in a number ofadvantages. The intermediate electrode may be kept at a specifiedelectrical potential, thereby avoiding disturbances of the detectorfunction by the potential of the connecting conductor. The limitationsof conventional conductor crossovers to small differences in potentialmay be overcome in an advantageous manner. The electrical connection ofthe connecting conductor to a measuring device by bonding may take placein the non-sensitive region of the detector at the edge of thesemiconductor substrate, at a distance from the anode. This simplifiesthe contacting and prevents detector failures resulting from substratedamage.

According to one advantageous embodiment of the invention, theintermediate electrode for electrical shielding of the semiconductorsubstrate completely overlaps the intermediate space between adjoiningsemiconductor electrodes. The width of the intermediate electrode ispreferably greater than the lateral distance between adjoiningsemiconductor electrodes. To simplify the component structure with asingle intermediate electrode, multiple intermediate spaces betweenadjoining semiconductor electrodes may be overlapped. Alternatively, itis possible for the intermediate electrode to have two or more parts,and to shield only one particular partial region of the intermediatespace between the adjoining semiconductor electrodes. Preferably, theintermediate electrode shields the crossover between the semiconductorsubstrate and a semiconductor electrode with its individual parts.

The electrical potential of the intermediate electrode preferably isdetermined by electrically connecting this electrode to one of theadjoining semiconductor electrodes, thereby keeping the difference inpotential between the intermediate electrode and the adjoiningsemiconductor electrodes low. Alternatively, the potential of theintermediate electrode may be determined by connecting the intermediateelectrode to an external power source.

The intermediate electrode is composed of an electrically conductivematerial such as metal, an intrinsic or doped semiconductor, or aresistor material, for example. Preferably, no particular demands areplaced on the conductivity of the intermediate electrode and optionallythe electrical connection of the intermediate electrode to one of thesemiconductor electrodes, since the intermediate electrode is usedsolely for electrostatic shielding. A poorly conductive compound may beprovided, since there is no need for the intermediate electrode toconduct electrical power. For example, an intermediate electrode made ofpolysilicon may be connected directly to one of the semiconductorelements made of silicon. The direct polysilicon-silicon crossover,which is otherwise avoided in semiconductor elements, simplifies thedesign of the detector according to the invention.

According to a further advantageous embodiment of the invention, inorder to improve the electrical shielding, multiple levels may beprovided with intermediate electrodes placed one on top of the other,the individual levels being insulated from one another by additionalinsulation layers. Such a multilayer arrangement of the intermediateelectrodes is particularly advantageous when the difference in potentialbetween the connecting conductor and the semiconductor electrodes isvery large (in the kV range, for example).

In one advantageous variant of the invention, at least one shieldingelectrode having a fixed electrical potential is also provided which issituated between the connecting conductor and the semiconductorsubstrate. When the signal-emitting readout electrode (anode) iscentrally positioned on the semiconductor substrate, the shieldingelectrode (at least one) is preferably situated on the outer edge of thesemiconductor substrate. To improve the electrical shielding, multipleshielding electrodes may be provided, each having a fixed electricalpotential.

Guiding of the connecting conductor according to the invention is notlimited to the contacting of the readout electrode, but may also besimilarly used for contacting the individual semiconductor electrodes.In this manner all connecting conductors for the readout electrodes aswell as for the semiconductor electrodes may be guided outward, thusenabling simple and mechanically secure contacting. If the readoutelectrode is integrated together with an amplifier element (atransistor, for example) onto the semiconductor substrate, the conductorcrossover according to the invention may also be used for connecting theamplifier element, for example, for the connector contacts of thetransistor. Multiple connecting conductors may be shielded with oneintermediate electrode.

The invention is not limited to specific doping ratios in the detector.For example, a weakly n-doped semiconductor substrate with stronglyp-doped semiconductor electrodes and a readout electrode of stronglyn-doped semiconductor material, or conversely, a weakly p-dopedsemiconductor substrate with strongly n-doped semiconductor electrodesand a strongly p-doped readout electrode, may be provided.

In an X-ray drift detector, the semiconductor substrate should be soweakly doped that it is fully depleted in normal operation so that nofree charge carriers are present in the non-irradiated semiconductorsubstrate. Correspondingly, the semiconductor electrodes and the readoutelectrode should be made of a material that is so strongly doped thatfull depletion does not occur in normal operation.

The conversion according to the invention is not limited to the use ofsilicon or polysilicon as semiconductor material. Rather, the individualsemiconductor components may also be produced from other materials, forexample germanium or gallium-arsenide. The use of silicon assemiconductor material, however, offers the advantage of economicalavailability and a fully developed technology.

According to a further advantageous embodiment of the invention, the atleast one connecting conductor is guided over semiconductor electrodeswhich have an annular topology. An annular topology is formed, forexample, by electrodes that are circular, oval, or polygonal, or thatrun in a non-concentrically closed manner. Multiple semiconductorelectrodes which mutually enclose one another may be provided.

An additional advantage of the conductor crossover according to theinvention concerns the combination of semiconductor detectors in groups.Large-surface detector assemblies or arrays, for example having numeroushoneycomb-shaped adjoining drift detectors, may be more easilycontacted, since the connecting conductors comprising multiple driftdetectors are guided at the edge of the detector assembly and may becontacted there. Connecting conductors may be guided not only via onedrift detector at its edge, but also may be guided via a plurality ofdetectors without impairing their function. In addition, guiding theconnecting conductors according to the invention allows a closelystacked arrangement of multiple drift detectors above one anotherwithout interfering scattering or absorption material being situatedbetween the individual drift detectors.

For a detector array, each semiconductor substrate preferably has theshape of a hexagonal disk, thus enabling numerous detectors to beadjacently positioned next to one another without gaps. Thesemiconductor electrodes preferably run in a manner corresponding to ahexagonal ring. The hexagonal shape is the preferred polygonal shapewhich most closely approximates a circular surface, thereby providing alarge detector surface and also enabling a closely packed arrangement onone level, without overlapping. The monolithic integration of numeroushexagonal drift detectors on one wafer is particularly advantageous.

Alternatively, it is possible for the semiconductor substrate to havethe shape of a circular disk, whereby the semiconductor electrodespreferably have an annular shape. Lastly, it is also possible for theindividual semiconductor electrodes to have a linear arrangement and torun parallel to one another, the readout electrode then preferably beingsituated next to the semiconductor electrodes.

Further advantageous refinements of the invention are characterized inthe subclaims, or are explained in greater detail below along with thedescription of the preferred embodiment examples with reference to thedrawings.

FIG. 1 shows a cross-sectional illustration of a drift detector with afirst exemplary embodiment of a conductor crossover according to theinvention;

FIG. 2 shows an illustration of a detector with an alternative exemplaryembodiment of a conductor crossover having additional shieldingelectrodes;

FIG. 3 shows an illustration of a detector with an alternative exemplaryembodiment of a conductor crossover in which multiple levels ofintermediate electrodes are superposed; and

FIG. 4 shows a top view of a drift detector having multiple conductorcrossovers according to the invention.

FIG. 1 shows a silicon drift detector in an enlarged cross-sectionalview, which may be used in X-ray spectroscopy, for example. Thecylindrical drift detector comprises a semiconductor substrate 1, on thesurface of which doped regions form semiconductor electrodes 2, 3, andA. Radiation detection is performed by the detection of electrons whichare released by the radiation in the semiconductor substrate 1, thesemiconductor electrodes interacting in a manner known as such, asdescribed for example by L. Strüder in the above-referenced publication.The semiconductor substrate 1 comprises an n-doped silicon disk, thedoping being so weak that the semiconductor substrate 1 is totallydepleted in the sensitive region. The semiconductor substrate 1 has, forexample, a thickness of approximately 300 μm and a diameter of severalmm, depending on the application, such as approximately 2.5 mm, or up toseveral cm, for example 10 cm.

The readout electrode A, which is made out of an n-doped semiconductormaterial, is centrally positioned on the top side of the semiconductorsubstrate 1 (with reference to FIG. 1). The doping of the readoutelectrode A is so strong that total depletion does not occur duringoperation. The low capacitance of the readout electrode A in theillustrated arrangement is advantageous, so that even radiation havinglow energy and short duration of effect can be spectroscopicallydetected.

Multiple annular semiconductor electrodes 2 are situated on the top sideof the semiconductor substrate 1 which concentrically surround thereadout electrode A. Each of the individual semiconductor electrodes 2is composed of a p-doped semiconductor material, whereby the doping ofthe semiconductor electrodes 2 is so strong that total depletion doesnot occur during operation. The semiconductor electrodes are alsoreferred to as field rings R1 through Rn.

A first insulation layer Ox is provided on the top side of thesemiconductor substrate 1 which leaves the readout electrode A and,optionally, parts of the semiconductor electrodes 2 open for productionof electrical connections. A flat counterelectrode 3 is situated on thebottom side of the semiconductor substrate 1, opposite from the readoutelectrode A and the semiconductor electrodes 2. The counterelectrode 3is composed of a p-doped semiconductor material, the doping being sostrong that total depletion of the counterelectrode 3 does not occurduring operation.

The individual semiconductor electrodes 2 are acted on from the centerto the edge with an increasingly negative electrical potential, therebyforming the above-referenced electrical drift field inside thesemiconductor substrate 1 which drives the electrons released throughradiation in the semiconductor substrate 1 in the direction of thereadout electrode A. The minima of the potential lines of the electronpotential lie on a curve 4, along which the released electrons migratetoward the readout electrode A.

The semiconductor electrodes 2 are controlled by an integrated voltagedivider (see left half of FIG. 1), comprising a chain of MOS enrichmenttransistors, for example, which is operated in the direction of passage.The width of the individual MOS enrichment transistors may be smallcompared to the circumference of the annular semiconductor electrodes 2.Each of the MOS enrichment transistors is formed by crossing a conductorbetween adjoining semiconductor electrodes 2, over the insulation layerOx, which is situated on the top side of the semiconductor substrate 1,so that the voltage drops to approximately the threshold voltage of thetransistor between the individual semiconductor electrodes 2.Alternatively, a resistive voltage divider or a punch-through structuremay be provided for controlling the semiconductor electrodes.

The contacting of the readout electrode A with the conductor crossoveraccording to the invention is described below. The readout electrode Ais connected to a connecting conductor M which is outwardly guided overthe annular semiconductor electrodes 2 to a bond pad B. Multipleintermediate electrodes L are located between the connecting conductor Mand the semiconductor electrodes 2. The intermediate electrodes L arestructured partial layers having a surface which in the radial directioncompletely covers the intermediate space between each pair of adjoiningsemiconductor electrodes, and in the transverse direction is at least aswide as the connecting conductor M. The radial extension of theintermediate electrodes L is preferably selected so that thesemiconductor electrodes 2 overlap in a perpendicular projection,whereby the overlapping is two to three times, for example, thethickness of the insulation layer Ox. The semiconductor substrate 1 iselectrically shielded by the intermediate electrode L in order toprevent charge carrier amplification or electrical breakthroughs in thesemiconductor substrate 1 between the semiconductor electrodes 2 onaccount of the field effect of the connecting conductor M. An additionalinsulation layer I is present between the connecting conductor M and theindividual intermediate electrodes L in order to insulate the connectingconductor M from the individual intermediate electrodes.

The individual intermediate electrodes L are each kept at a fixedelectrical potential by electrically connecting each individualintermediate electrode L to one of the two adjoining semiconductorelectrodes.

Lastly, the drift detector has an external electrode 5 which preferablyis guided outwardly past the bond pad B.

The exemplary embodiments of inventive conductor crossovers illustratedin FIGS. 2 and 3 are substantially similar to the exemplary embodimentdescribed above, so that the same reference numbers are used forcorresponding elements. To avoid repetition, reference is made to thedescription for FIG. 1.

The exemplary embodiment according to FIG. 2 is distinctive in that onthe top side of the semiconductor substrate 1, in the outer region,multiple annular, concentrically arranged shielding electrodes G₁-G_(m)are provided which are set at an electrical potential which outwardlydecreases from the inside to the outside until the potential of asubstrate electrode S is reached which determines the potential of thesemiconductor substrate in the edge region.

The exemplary embodiment according to FIG. 3 is distinctive in that twolevels of intermediate electrodes L, L₂ are situated between theconnecting conductor M and the individual semiconductor electrodes 2.The multilayer arrangement of intermediate electrodes L, L₂ preventsdisturbances of the detector function by the field of the connectingconductor M, even at very high voltages between the connecting conductorM and the semiconductor electrodes 2.

An additional insulation layer I₂ is situated between the connectingconductor M and the additional intermediate electrode L₂ in order toinsulate the connecting conductor M from the intermediate electrode L₂.

FIG. 4 shows a drift detector in top view, in which the readoutelectrode is integrated with a field effect transistor onto thesemiconductor substrate. In the top view, the p-doped regions(semiconductor electrodes 2, 2 a, 2 z, 2 i, gate G) are shaded, and then-doped regions (free areas of the semiconductor substrate 1, drain D,source S) are unshaded. The transistor Tr comprises the n-doped centraldrain region D, the p-doped annular gate region G, and the n-dopedannular source region S. The gate region G is connected to the anode A(readout electrode). Each of the connection contacts of the transistorTr is individually guided, using a conductor crossover according to theinvention, to the outside edge of the drift detector. Furthermore,semiconductor electrodes 2 are connected as a group with conductorcrossovers to external contact sites (bond pads). By use of theintermediate electrodes L according to the invention, connectingconductors may be shielded individually or in groups.

In the illustrated example, the bond pads B1 through B6 arecorrespondingly connected to the source S, drain D, the innermostsemiconductor electrode 2 i, an intermediate semiconductor electrode 2z, an external semiconductor electrode 2 a, and a p-doped insulationring Is between the source S and the anode A. The two outermost ringsare shielding electrodes (guard electrodes) which are used to lower thevoltage and which are not contacted. A substrate contact (notillustrated) is provided outside the outermost ring.

The invention is not limited to the above-described exemplaryembodiment. Rather, a number of variants and modifications areconceivable which make use of the inventive concept and thereforelikewise fall under the protection of the invention.

1. A conductor crossover for a semiconductor detector that is capable ofbeing used with a drift detector for X-ray spectroscopy, comprising: atleast two doped semiconductor electrodes situated in a semiconductorsubstrate, at least one connecting conductor guided over thesemiconductor electrodes, and a first insulation layer, wherein betweenthe connecting conductor and the first insulation layer an intermediateelectrode is situated which covers the region of the semiconductorsubstrate between the semiconductor electrodes and which is electricallyinsulated from the connecting conductor by at least one additionalinsulation layer.
 2. The conductor crossover according to claim 1,wherein the intermediate electrode is electrically connected to at leastone of the semiconductor electrodes and has the same electricalpotential as said semiconductor electrode.
 3. The conductor crossoveraccording to claim 2, in which the connection between the intermediateelectrode and the semiconductor electrode is formed by apolysilicon-silicon crossover.
 4. The conductor crossover according toclaim 1, wherein the intermediate electrode is connected to an externalpower source in order to set its electrical potential.
 5. The conductorcrossover according to one claims 1-4, wherein the conductor crossoverhas multiple levels containing a plurality of insulated intermediateelectrodes situated, one above the other, between the connectingconductor and the semiconductor substrate.
 6. The conductor crossoveraccording to one of claims 1-4, comprising at least one additionalconnecting conductor, which is guided over adjoining semiconductorelectrodes, for contacting the semiconductor electrodes.
 7. Theconductor crossover according to one of claims 1-4, wherein thesemiconductor electrodes are p-doped, and the semiconductor substrate isn-doped.
 8. The conductor crossover according to one of claims 1-4,wherein the semiconductor electrodes are n-doped, and the semiconductorsubstrate is p-doped.
 9. The conductor crossover according to one ofclaims 1-4, wherein the semiconductor substrate is made essentially fromone of the group consisting of silicon, polysilicon, germanium andgallium-arsenide.
 10. The conductor crossover according to one of claims1-4, wherein the connecting conductor is guided over the semiconductorelectrodes which have an annular topology.
 11. The conductor crossoveraccording to claim 10, wherein the connecting conductor is guided overmultiple semiconductor electrodes which mutually surround one another.12. The conductor crossover according to one of claims 1-4, wherein theat least one connecting conductor is guided over multiple adjoiningdrift detectors.
 13. A drift detector for X-ray spectroscopy having atleast one conductor crossover according to one of claims 1-4.
 14. Adetector assembly for X-ray spectroscopy, comprising multiple driftdetectors and having at least one conductor crossover according to oneof claims 1-4, which is guided over at least two of the multiple driftdetectors.
 15. The conductor crossover according to one of claims 1-4,comprising at least one additional connecting conductor, which is guidedover adjoining semiconductor electrodes, for contacting an amplificationelement.
 16. The conductor crossover according to one of claims 1-4,additionally comprising a readout electrode, wherein the semiconductorelectrodes are p-doped and the readout electrode is n-doped.
 17. Theconductor crossover according to one of claims 1-4, wherein thesemiconductor electrodes are made essentially from at least one of thegroup consisting of silicon, polysilicon, germanium andgallium-arsenide.
 18. The conductor crossover according to one of claims1-4, additionally comprising a substrate electrode, wherein thesubstrate electrode is made essentially from at least one of the groupconsisting of silicon, polysilicon, germanium and gallium-arsenide.